Mobile equipment such as a road vehicle fitted with communications equipment for military purposes has mains filters which result in large earth leakage currents, which will cause the vehicle chassis to become live in the event of mains earth failure. Conventional earth leakage detectors are not suitable because of the large earth leakage currents and voltage trip breakers require a separate reference earth and are often over sensitive to external vibration so that they trip out too frequently and in consequence are by-passed or shorted out in use. The invention aims to provide simple electric shock protection for users of mobile equipment.
According to the invention protective means for detecting the existence of an unsafe potential between earth and mobile equipment to which an AC voltage source is applied, comprises an electrically conductive member disposed between the equipment and earth, means for detecting the electric field strength between the mobile equipment and earth at the location of the conductive member and thereby deriving a measure of the potential difference between the mobile equipment and earth, and means responsive to a dangerous condition corresponding to a dangerous potential of the mobile equipment with respect to earth.
Said means for detecting the potential of the electrically conductive member with respect to the mobile equipment preferably comprise a high impedance amplifier and the means responsive to a dangerous condition preferably comprise a power trip for switching out the voltage source to the mobile equipment, or some form of alarm such as an audible or visual alarm.
The electrically conductive member is preferably a metal plate which is fixed with respect to the mobile equipment and which faces the ground, the metal member being spaced both from a chassis part of the mobile equipment and the ground so that the potential of the metal member is representative of the potential of the mobile equipment, the precise relationship depending on the relative values of the capacitances between the metal member and the mobile equipment and between the metal member and earth. It will be appreciated that the capacitances will depend on the area of the plate, the spacings between the mobile equipment and the metal member and between the metal member and the ground, and the dielectric constants of these spacings.
In a preferred embodiment the metal plate is bonded to a plastics material which is itself bonded to the underside of the equipment. In another embodiment the metal plate is included in a detector unit which is recessed into an aperture formed in the base of the mobile equipment, the detector plate being sealed and protected against the ingress of dirt or moisture into the detector unit.
The mobile equipment may be a wheeled or tracked road vehicle, such as a lorry or tank, or may be a mobile cabin or container.